BA's website is not very helpful for situations like this. As others have pointed out, as a passenger you a) must be aware that such a thing (being put into economy on a business class fare) is even possible, b) check each flight details in order to see which cabin you ended up in. And then, as you correctly pointed out, there is no option to select a more expensive business class fare where all of the segments are in business class (if that is your preference).

This compares very negatively with American Airlines website, for example. There you are given a warning that one of your flights is in the lower cabin and you are given an option to buy a ticket where all of the flights are in the premium cabin.